A standard way to link stones on the second line that are three points apart is to play kosumi on the first line from either:
connects at
.
Although both moves connect, Sakata points out in Tesuji and Anti-Suji of Go, that b in the first diagram is clearly preferable to a.
The analysis is based on the worst case where White is forced to allow Black to play twice in a row (most likely because of a ko fight). For instance :
( elsewhere, resolves a ko)
The marked white stone is a wasted move.
A little problem: